Most contemporary New Madrid seismic zone earthquakes correlate geographically with the location of a basement uplift, the Pascola arch, and a distinct magnetic anomaly called the Reelfoot magnetic anomaly (RMA). We invert magnetic data for several profiles crossing the RMA and demonstrate that the anomaly is produced by an igneous intrusion related to the formation of the Pascola arch. The models require approximately 2 km of vertical uplift on crystalline basement rock, indicating that structure associated with the Pascola arch extends below Paleozoic rocks. The modeled uplift corresponds to estimates of Paleozoic sediment erosion from the top of the arch. We suggest that faulting accompanied emplacement of the RMA intrusion, establishing a zone of weakness in the upper crust that manifests today as the Reelfoot fault. Faulting accompanying intrusion provides an explanation for why earthquakes occur within the RMA intrusion rather than surrounding it, as is the case with all other intrusions emplaced along the margins and axis of the Reelfoot graben.